The present invention relates to permanent magnetic couplers of the type having a permanent magnet rotor assembly on one shaft spaced by air gaps from a conductor rotor assembly on another shaft.
An adjustable, permanent magnetic coupler is known in which a pair of conductor rotors straddles a pair of magnet rotors. The conductor rotors are connected together to rotate as a unit on one shaft, while the magnet rotors are mounted to rotate with a second shaft and to be axially movable relative to the second shaft. Each magnet rotor has a set of permanent magnets spaced by an air gap from a ferrous-backed electroconductive ring mounted on a respective one of the conductor rotors. Rotation of one of the two shafts results in rotation of the other shaft by way of magnetic forces acting between the magnet rotors and the conductor rotors, without any direct mechanical connection between the shafts.
By moving the magnet rotors axially with respect to the second shaft, the air gap between the magnet rotors and the conductor rotors can be changed. Increasing the air gap reduces the torque coupling the magnet rotors and the conductor rotors. Knowing the relationship between the air gap and the resultant torque, engineers can design an adjustable magnet coupler that converts a particular incoming rotational speed and/or torque to a desired output speed and/or torque by creating the appropriate gap. Actuators have been designed to controllably move the magnet rotors with respect to the conductor rotors to adjust the air gaps in adjustable magnetic couplers.
Under certain operating conditions, such as during startup or upon substantial slowing of the load shaft, the relative rotational speeds of the magnet rotors and the conductor rotors can differ substantially. It has been reported that, under high slip conditions, a repulsion force is generated between the magnet rotors and the conductor rotors. The repulsion force is a function of relative movement slip and, thus, reduces as the rotors approach the same rotational velocity. Under relative static conditions, there is no repulsion force.
The present invention is directed toward a magnetic coupler configured to allow limited disengagement during startup, but to allow significant disengagement upon emergency stoppage. In one embodiment of the invention, a pair of magnet rotors is slidably mounted on torque rods extending between two end plates. Ferrous material on the conductor rotors is magnetically attracted to the permanent magnets in the magnet rotors. Under static or relative static conditions, the attractive force urges the magnet rotors toward the conductor rotors. Stops retain the magnet rotors apart from the conductor rotors by a minimum air gap selected for desired operating conditions. In certain embodiments, the stops are adjustable to change the minimum air gap.
During startup or emergency stoppage, however, relative rotational velocity between the magnet rotors and the conductor rotors causes an increase in the repulsion force between the magnet rotors and the conductor rotors. The attractive magnetic force is designed to be small enough to be overcome by the repulsion force during start up and emergency stoppage conditions. As a result, under such conditions the repulsion force moves the magnet rotors apart from the conductor rotors. During startup, the magnet rotors collapse against a latch arm positioned therebetween, increasing the air gap between the magnet rotors and the conductor rotors from the minimum air gap to a xe2x80x9csoft-startxe2x80x9d air gap. The soft-start air gap allows the load to come up to operating speed at a reduced torque level, reducing the impact on the equipment associated with startup. The size of the latch arm or other structures therebetween can be changed to adjust the soft-start air gap.
When the shaft on the load side approaches the rotational speed of the shaft connected to the motor, the repulsion force decreases to a point where it is again overcome by the attractive magnetic force between the magnet rotors and the conductor rotors. Under these conditions, the magnet rotors move back into the operational configuration, spaced by the minimum air gap from the conductor rotors.
At a pre-selected rotational velocity, centrifugal force moves the latch arms from the soft-start configuration to a xe2x80x9crunning speedxe2x80x9d or fully disengaged configuration. While the latches are in the running speed configuration, should the load shaft rapidly decelerate, the differential rotational velocities between the magnet rotors and the conductor rotors will result in the above-discussed repulsion force. Because the latch is now in the running speed configuration, however, the magnet rotors are able to collapse beyond the soft-start air gap. The magnet rotors instead collapse further, creating a maximum air gap between the magnet rotors and the conductor rotors that can substantially reduce the rotational torque forces therebetween. This fully disengaged configuration can allow the motor to continue running indefinitely while the load has substantially slowed or ceased, and can thus prevent damage to the equipment.
Ultimately, when the motor and the load have both stopped, the repulsive force again diminishes, allowing the attractive magnetic force to draw the magnet rotors back out to the initial, static configuration.
As discussed in more detail below, the present invention is directed toward a number of structures that allow the magnetic coupler to freely move between the above discussed configurations and, as such, to allow the system to automatically assume distinct configurations for startup, standard operating conditions, emergency stoppage, and static conditions. The present invention is also directed toward methods of performing the same.